To cope with the recent strong demand for safety and quality maintenance of foods, it has been a practice to make the inner atmosphere of food packages oxygen-free to minimize deterioration of foods by oxidation. More specifically, a deoxidizer that absorbs oxygen in the environment is co-packaged with a food so that the deoxidizer removes residual oxygen inside the package thereby to make the inner atmosphere oxygen-free. Furthermore, co-packaging a food and a deoxidizer is carried out in an oxygen-free inert gas atmosphere so as to prevent oxygen entrapment into the package, and a trace amount of oxygen that may penetrate through the packaging material is removed by the co-packaged deoxidizer.
Known deoxidizers used to remove oxygen in an environment include organic materials and inorganic materials. Iron-based deoxidizers, which are inorganic materials, are predominant from the economical standpoint. Iron-based deoxidizers react with environmental oxygen and a trace amount of water in the environment according to chemical formula (1) below thereby to remove oxygen from the environment.Fe+1/2H2O+3/4O2→FeOOH   (1)
Use of conventional iron-based deoxidizers as described involves the following problems.
(1) Since they need the presence of water in reacting with oxygen, a moisture retaining agent may be incorporated. Such a deoxidizer releases moisture to increase the humidity in the package. Therefore, when, for example, a tablet is sealed in an air-tight bag made of gas barrier film, a dehumidifier, e.g., silica gel should be co-packaged in some cases in order to maintain the dry state in the package. In such cases, the step of sealing a tablet requires means for putting a deoxidizer and means for putting a dehumidifier, which complicates the step. Moreover, removal of moisture from inside the bag with a dehumidifier reduces the water content necessary to induce the oxidation reaction, resulting in reduction of the deoxidizing function of the deoxidizer.(2) Co-packaging a moisture-retaining agent is not permitted in sealing a pharmaceutical, supplement, or other tablet that must be maintained in a highly moisture-free environment. So, oxidation promotion by moisture represented by formula (1) above is not expected, and the deoxidizing function is not fully exerted. As a result, a larger amount of iron powder than usual would be needed. The same problem also arise in sealing and storing other products than pharmaceutical or supplement tablets that should be stored in the absence of moisture, such as dry foods, electronic components, and solder powder.(3) When a food package having a food and an iron-based deoxidizer co-packaged is inspected for the presence of foreign matter such as metal, the iron powder of the deoxidizer would set off a metal detector, providing a hindrance to carrying out simple inspection.(4) An iron-based deoxidizer can heat up abruptly and ignite when microwaved, e.g., in a microwave oven.
Deoxidizers based on inorganic oxides having been subjected to a reduction treatment have been proposed in place of the iron-based deoxidizers. For example, Patent Document 1 and Patent Document 2 disclose titanium dioxide, zinc oxide, and cerium oxide as exemplary inorganic oxides having been subjected to a reduction treatment. These inorganic oxides are used in combination with an organic or inorganic compound capable of retaining moisture. On releasing moisture, the organic or inorganic compound capable of retaining moisture serves as a trigger for the inorganic oxide having been subjected to a reduction treatment to absorb oxygen. In order words, the deoxidizer according to the patent documents supra needs the presence of moisture in the environment to cause the inorganic oxide having been subjected to a reduction treatment to absorb oxygen, with no considerations given to dehumidification of the environment. In fact, titanium dioxide, zinc oxide, cerium oxide, and the like do not perform sufficient dehumidifying function in terms, e.g., of rate of dehumidification and the lowest possible humidity when used alone.
Patent Document 1 JP 2005-104064A
Patent Document 2 JP 2005-105195A